Conventionally, transparent materials having high dielectric constants such as glasses and plastics tend to accumulate static electricity and allow transmission of electromagnetic waves. In particular, in cathode ray tubes and plasma displays which are often used for TV Braun tubes and computer displays, dust can collect due to static electricity generated on the display screen so as to lower the visibility, and electromagnetic waves can be radiated to affect the environs. For this reason, transparent conductive films have been affixed to the display screens of cathode ray tubes and plasma displays used in TV Braun tubes and computer displays for the purposes of preventing static electricity and/or shielding electromagnetic waves.
Conventional transparent conductive films are produced by forming a transparent conductive oxide such as indium oxide on a display screen by means of sputtering or vapor deposition and affixing this to the display screen of a display device, or by coating the front surface of a display screen with a dispersion fluid of antimony-doped tin oxide and a silica sol binder. Additionally, there are transparent conductive films which are provided with a reflection preventing function by making use of interference effects which occur due to reflection at a plurality of thin film surfaces, by laminating the top layer and/or the bottom layer of the transparent conductive film with at least one transparent anti-reflective layer having a refractive index different from the transparent conductive layer.
As conventional methods for forming a transparent conductive film which not only prevents the accumulation of static electricity but also has conductivity high enough to shield electromagnetic radiation on the display screen of a display device, a process of putting the display screen into a vapor deposition oven and depositing indium oxide compounds or tin oxide compounds thereon (PVD process), and a process of forming a transparent conductive film on the display screen by thermal decomposition of organic compounds such as indium or tin, or saline solution (CVD process) are known.
While the transparent conductive films formed by the above-mentioned methods are sufficiently transparent when used only as anti-static films in which case the film thickness can be small, when used as electromagnet wave shielding layers or electrode films, they must be made somewhat thicker because they require high conductivity, as a result of which the transparency can be reduced, and the screen darkened, as well as giving rise to problems such as absorption of specific optical wavelengths which can cause coloring of the conductive film and cause unnatural changes in the hue of the transmitted images. Additionally, since a vacuum or high temperatures are required in order to form a film using the above-mentioned PVD process or CVD process, the cost of investments in order to form a transparent conductive film on a large-area substrate can become extremely high, the efficiency can be degraded, and the production costs can inflate.
Coating processes have been proposed for efficiently forming transparent conductive films on large substrates while suppressing equipment investments. For example, coating materials containing organic indium compounds is disclosed in Japanese Patent Application, First Publication No. Sho 52-1497, and coating materials having indium salts or tin salts dissolved in water or organic solvents are disclosed in Japanese Patent Application, First Publication No. Sho 63-6401, Japanese Patent Application, First Publication No. Sho 55-51737, Japanese Patent Application, First Publication No. Sho 58-82407, Japanese Patent Application, First Publication No. Sho 57-36714, and Japanese Patent Application, First Publication No. Sho 60-22507. However, since the formation of transparent conductive films using these coating materials requires heat treatment at high temperatures of at least 350.degree. C. after coating the substrate, there are limitations to the materials which can be used for the substrate, and there are many restrictions with regard to the production steps.
Coating materials wherein microparticles or colloids of transparent conductive oxides such as tin oxide and indium oxide are dispersed in polymer solutions or binder resins are disclosed in Japanese Patent Application, Second Publication No. Sho 35-6616, Japanese Patent Application, First Publication No. Sho 57-85866, Japanese Patent Application, First Publication No. Sho 58-91777, and Japanese Patent Application, First Publication No. Sho 62-278705. These coating materials are held to be capable of forming transparent conductive films at relatively low temperatures.
However, all of the above-mentioned transparent conductive films require the thickness of the coating film to be made small in order to obtain a practical level of transparency; making them thin causes the conductivity to be reduced, so that although they are useful for the purposes of static prevention, they are insufficient for the purposes of electrical shielding; making them thick causes the transparency to be reduced, which darkens the screen and limits the possibilities of use.
As a transparent conductive film having exceptional electromagnetic shielding effects and anti-reflection effects, Japanese Patent Application, First Publication No. Hei 8-77832 disclosed one comprising a transparent conductive layer composed of metallic microparticles having an average grain size of 2-20 nm and a transparent coat having a lower refractive index. Although electromagnetic shielding effects can be expected of this transparent conductive film, it fails to provide a solution to the problem of absorption occurring at specific wavelengths of transmitted light depending on the light transmission spectrum of the metal, thereby coloring the conductive film and unnaturally changing the hue of the transmitted image, and it also cannot be expected to provide sufficient anti-reflective effects.
Aside from the above, if the purpose is simply to form a conductive film, Japanese Patent Application, First Publication No. Hei 4-23484 discloses a process of coating with a coating material wherein reduced metallic colloid particles are dispersed in a photosensitive resin, and Japanese Patent Application, First Publication No. Hei 4-196009 discloses a process of printing a conductive paste onto a dielectric green sheet using a screen printing method, but these are both non-transparent and do not result in a transparent conductive film.
The present invention has been made to resolve the above problems, and therefore its object is to offer a transparent conductive film having high transparency, tonability, and conductivity, exceptional anti-static effects and electromagnetic shielding effects, adjusted tone of the transmitted images, and greatly improved durability such as with respect to saline resistance, acid resistance, oxidation resistance, and ultraviolet resistance; a low-reflectivity transparent conductive film having exceptional anti-reflective effects in addition to the above-mentioned properties; and an electromagnetically shielded display device having this low-reflectivity transparent conductive film formed on the display screen.